Enhanced assessment of technological factors for Ti-6Al-4V and Al-Cu-Mg strength properties

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 The fi rst group is the regular loading with fracture of samples at LCF (low-cycle fatigue) [2]. Cof fi n’s formula is widely used, which includes the characteristic of inelastic deformation – the width of the hysteresis loop [3]. The second group of methods is loading using the effect of increasing the test frequency. This technique initiated the study of “in fi nite durability” – 10 6 – 10 10 cycles ( Batias ) [4]. In this direction, researchers [5, 6] were the fi rst to pay attention to the fact that under the loading regime of more than 10 9 cycles, cracks occur under the surface of the sample [7]. The fracture nucleus looks like a “Fish-eye” [8, 9]. In the third group, loading is considered using a programmed load change: Lokati , Pro , Enomoto , Weibull [10–12]. The general disadvantage of the three groups of enhanced methods considered above: destruction of a large number of samples, large error, it is realistic to evaluate only cyclically stable materials, which limits the presented methods. And also, the above methods are not associated with the study of physical processes that occur in the structure of the material at different scale levels, under the in fl uence of external load. The fourth group includes cyclic loading without bringing the samples to failure. In indirect methods, the value of the fatigue endurance limit is associated with the characteristics of the mechanical properties or physical phenomena of metals that accompany the fatigue process. These methods are based on establishing the relationship between fatigue limits and stresses at which irreversible effects associated with fatigue damage begin to appear in the test material. The physical basis of non-destructive methods is structurally sensitive characteristics and accompanying phenomena occurring in the material during cyclic loading [21- 48]: phase transformations in the material [13-16], where transformations in alloys of the martensitic type are observed; microhardness [17], distortion of the crystal lattice of the metal [18], change in the characteristics of magnetic resistance, magnetic hysteresis, eddy currents [19-21], change in the surface relief [22, 23], acoustic emission [24], intensi fi cation of irreversible energy dissipation or inelastic deformations [22], changes in the microstructure [9, 23, 24], etc. There are methods in which the accumulation of damage is associated with a change in various integral characteristics of energy dissipation in the metal, based on the measurement of the absorption coef fi cient, the logarithmic decrement of oscillations, and the temperature in the fracture nucleus [25–30]. Themain purpose of the study is to detect physical phenomena accompanying the process of cyclic loading in the transition region from elastic to inelastic deformation, analyze energy dissipation and accumulation of deformations that occur during inelastic cyclic deformation at constant non-zero average stresses. At the same time, the in fl uence of technological impact on the determined characteristics is revealed. Further research is devoted to a discussion of approaches to these phenomena simulation. Materials and Methods Test samples A batch of samples for study was made from a sheet of high-strength titanium alloy VT6 ( Ti-6Al-4V ) and a sheet of aluminum alloy D16 ( Al-Cu-Mg ). The choice of these materials is due to the fact that both alloys are widely used in aircraft construction. D16 aluminum alloy has historically been the main material in the fi eld of aircraft construction. Titanium alloy VT6 is used, for example, for the manufacture of disks and blades of the fi rst stages of gas turbine engines. These materials are supplied in various forms: forgings, stampings, rods, plates and sheets. The history of the deformation of semi- fi nished products is created at the stage of its manufacture, in which a variety of technological processes affect the material: rolling, drawing, forging, machining, heat treatment, etc. Technological factors preceding the test of a material sample for fatigue failure resistance have a strong in fl uence on the durability of the sample. The study uses samples of type IV in accordance with GOST 25.502-79 (Fig. 1). The length of the working part of the sample is 50 and 45 mm, which makes it possible to install two extensometers for measuring axial and transverse deformations. Samples of VT6 alloy were divided into two series: smooth